My ex is using our child as an excuse to tell me how to live my life. I’ve just realised that I have started using grey rock. I’m still a bit wobbly, but there’s something to be said for not allowing him to see he’s upset me.

i did not know what grey rock was till your post – thank you. fortunately i am nc with my childs dad – for now. however, i have had to apply it where i work and like KIP has stated, it was not long till they bit.
best wishes to you*

*whateverapproach* look up grey rock. Its about not giving an animated response when your abuser is trying to upset you or start an argument, so you just respond with things like yes, no or okay. The idea is they get bored of no reaction and start to leave you alone.